1. Field of the Invention
The present invention relates to an optical system and an image pickup apparatus including the optical system.
2. Description of the Related Art
Recently, image pickup apparatuses (e.g., cameras), such as video cameras and digital still cameras, using solid-state image pickup devices have become smaller with increased image quality. Accordingly, demand has increased for small, high-image-quality optical systems for use in such cameras.
In conventional structures, one method for reducing the size of an optical system, reduces the number of lenses included in the optical system.
However, if the number of lenses is reduced, it becomes difficult to correct aberrations like spherical aberration and coma aberration that affect monochromatic imaging performance. In addition, the kinds of glass materials that can be used are limited, and it can be extremely difficult to correct chromatic aberration.
In addition, when the number of lenses is reduced, the refractive power of each lens included in the optical system is relatively increased. Therefore, sensitivity, which is a change in the optical performance with respect to manufacturing errors, is increased and it can become necessary to increase the processing accuracy of each lens and the assembly accuracy of the optical system.
Accordingly, an optical system including an aspherical lens surface is commonly used in order to maintain good imaging performance even when the number of lenses is reduced.
On the other hand, optical systems in which cemented lenses with aspherical cemented surfaces are used to obtain high imaging performance and low sensitivity with a small number of lenses have been discussed (Japanese Patent Publication No. 4-5362, Japanese Patent Laid-Open No. 63-27809, Japanese Patent Laid-Open No. 2001-42212, Japanese Patent Laid-Open No. 2002-6210, and Japanese Patent Laid-Open No. 2004-61519).
However, when aspherical surfaces are used, even though aberrations that affect the monochromatic imaging performance can be corrected, chromatic aberrations, which are mainly affected by the selection of glass material, become difficult to correct.
A method for reducing the occurrence of the chromatic aberration by using an anomalous dispersion material as an optical material is known. In addition, optical systems including diffractive optical elements for correcting the chromatic aberration instead of using the anomalous dispersion material are also known (Japanese Patent Laid-Open Nos. 6-324262 and No. 6-331887).
In the diffractive optical elements, the absolute value of a numerical value corresponding to the Abbe number is generally small (around 3.45). The diffractive optical elements are characterized by being capable of largely changing the chromatic aberration while barely affecting the spherical aberration, the comma aberration, and astigmatism by only slightly changing the refractive power obtained by diffraction.
In addition, since the incident light is diffracted, the refractive power linearly varies with respect to the variation in the wavelength of the incident light. Accordingly, the wavelength characteristic of the chromatic aberration coefficient is completely linear.
Therefore, in an optical system including a diffractive optical element, when the total lens length is reduced to reduce the overall size of the optical system, it can become necessary to only correct the spherical aberration, the coma aberration, and the astigmatism. With regard to the chromatic aberration, it may not be necessary to consider the absolute value thereof as long as the glass materials and refractive powers of the lenses are optimally designed such that the wavelength characteristic of the chromatic aberration coefficient is linear. Thus, the optical system in which the total lens length is reduced can be obtained.
On the other hand, as an optical material having a chromatic-aberration-correcting function similar to the optical characteristics of the diffractive optical elements, a liquid material having relatively high dispersion and relatively anomalous dispersion characteristics is known. Recently, achromatic optical systems using such a material have been suggested (U.S. Pat. Nos. 5,731,907 and 5,638,215).
In addition, Japanese Patent Publication No. 4-5362 and Japanese Patent Laid-Open No. 63-27809 discuss the optical systems in which aspherical cemented surfaces are used to obtain high imaging performance and low sensitivity with a small number of lenses.
However, in each of the embodiments of the above-mentioned publications, the cemented surface is regarded as a single surface. Therefore, in order to carry out the embodiments, aspherical surfaces having exactly the same shapes as those described in the embodiments can be manufactured, and this can be difficult in view of both processing accuracy and complexity of manufacturing.
Japanese Patent Laid-Open No. 2001-42212 discusses a practical technique for cementing an aspherical surface and a spherical surface together in which an adhesive layer is taken into account.
However, in the cementing step, it can be necessary to center the aspherical surface and the spherical surface at the cementing portion in order to ensure the imaging performance. Therefore, the manufacturing process can be extremely difficult.
Japanese Patent Laid-Open Nos. 2002-6210 and 2004-61519 discuss a technique of laminating a resin layer on an aspherical surface of a lens composed of a glass material. Accordingly, the aspherical surface and a spherical surface can be substantially cemented together. However, this technique is designed for a pickup objective lens, and only the spherical aberration and the chromatic aberration with respect to two wavelengths are mainly corrected. Therefore, corrections of the field curvature and the chromatic aberration caused by white light, which can be necessary in imaging optical systems, are not provided.
The diffractive optical element provides a sufficient chromatic-aberration-correcting function as a method for correcting the chromatic aberration. However, the diffractive optical element also generates unnecessary diffracted light with diffraction orders other than that of the diffracted light used in practice. The unnecessary diffracted light functions as colored flare light that degrades the imaging performance. In order to reduce the unnecessary diffracted light, a so-called layered diffractive optical element in which a plurality of blaze diffraction gratings are laminated along an optical axis can be used.
Accordingly, the energy can be concentrated at the designed diffraction order and unnecessary diffracted light can be greatly reduced. However, when high-luminance objects are shot, flare is still generated because of the unnecessary diffracted light.
As a method for manufacturing the diffractive optical element, a method of forming the diffractive optical element composed of an ultraviolet curable resin by molding using a mold is known. However, according to this method, the sensitivity of diffraction efficiency of the diffractive optical element is extremely high. Therefore, a high-accuracy mold must be used and high molding accuracy is required.
The materials discussed in Japanese Patent Laid-Open Nos. 6-324262 and 6-331887 are liquid, and therefore a structure for sealing the liquid is required. In addition, the manufacturing process becomes difficult when the materials are used as optical materials.
In addition, characteristics including the refractive index, dispersion, etc., largely vary along with the temperature variation and the environment resistance is not sufficient. Furthermore, since an interface with air cannot be obtained, it is difficult to sufficiently correct the chromatic aberration.